This invention relates to a six-axis force sensor, and particularly to a small and highly precise six-axis force sensor in which strain resistance devices fabricated using semiconductor manufacturing process technology are used and which can detect six force and moment components and be utilized as a force-sensing sensor of a robot or the like.
Modern automatic machines such as machine tools and robots, in the course of their work or operation, perform work in which they apply forces to workpieces, or are themselves subject to actions of forces from outside. Consequently, it is necessary for machine tools and robots to detect forces and moments acting on them from outside and to perform control corresponding to these forces and moments. For control corresponding to external forces and moments to be carried out properly, it is necessary for the forces and moments acting from outside to be detected exactly.
In this connection, various types of multi-axis force sensors to be used as force-sensing sensors and man-machine interfaces have been proposed. Generally, force sensors can be classified, according to the detection method that they use, as either elastic-type force sensors, which detect a deformation proportional to a force, or force-balance-type force sensors, which measure a force by balancing it with a known force. As a principle structure, force sensors generally have multiple strain resistance devices provided on a distorting body part which deforms elastically in correspondence with external forces.
With this structure, when an external force acts on a distorting body part of the multi-axis force sensor, electrical signals corresponding to degrees of deformation of the distorting body part are outputted from the strain resistance devices. On the basis of these electrical signals it is possible to detect two or more force or moment components acting on the distorting body part.
To keep up with size reductions of devices equipped with multi-axis force sensors, size reductions of multi-axis force sensors themselves are sought. Accordingly, there is an increasing need for multi-axis force sensors which have good sensitivity and high precision while being small.
A typical multi-axis force sensor is the six-axis force sensor. The six-axis force sensor is a force sensor of the elastic type mentioned above, and has multiple strain resistance devices on a distorting body part. The six-axis force sensor divides an external force into axial stress components (forces Fx, Fy, Fz) in the axis directions and torque components (torques Mx, My, Mz) about the axis directions of three orthogonal coordinate axes (an X-axis, a Y-axis, a Z-axis), and detects it as six axis components.
A first example of a multi-axis force sensor in related art is the xe2x80x98Multiple Force Component Load Cellxe2x80x99 disclosed in JP-B-S.63-61609 (Publication date Nov. 29, 1988; corresponding U.S. Pat. No. 4,448,083). This document discloses a six-axis force sensor. This six-axis force sensor has a construction wherein multiple strain gauges are affixed to a distorting body having a solid (three-dimensional) structure. This sensor is a six-component force sensor and has a structure such that mutual interference arising among the six force components detected is reduced. The six-axis force sensor is made up of a central force-receiving part, a fixed annular part around this, and between these, four T-shaped connecting parts equally spaced around the axis of the force-receiving part. The strain gauges are affixed to low-rigidity portions of the beams of the T-shaped connecting parts.
With this structure wherein strain gauges are affixed to a distorting body, size reduction is limited; manufacturing reproducibility is poor and dispersion arises among units; and also problems such as peeling of the affixing layer arise due to repeated shock stresses and thermal stresses. When peeling of the affixing layer occurs, the measuring precision deteriorates. Alignment deviation also causes the measuring precision to deteriorate. The problem arises that it is difficult to make the mounting positions accurate enough to ensure good detection accuracy.
A second example of a multi-axis force sensor in related art is the xe2x80x98Two-or-more-Component Force-Detecting Devicexe2x80x99 disclosed in Japanese Patent Publication No. 2746298 (Published May 6, 1998, corresponding U.S. Pat. No. 4,884,083). In the multi-axis force sensor disclosed in this document, multiple strain resistance devices are fabricated on a substrate using semiconductor manufacturing process technology, and a strain gauge element is assembled integrally to a distorting body part. The substrate is made up of a peripheral part and a central part. According to this document, the problems of the multi-axis force sensor of the first related art mentioned above can be resolved, the precision of the fabrication process can be raised, the reproducibility of fabrication can be made good, and the multi-axis force sensor can be reduced in size. However, with this sensor, there is a high probability of mutual interference arising among the six axis components detected.
A third example of a multi-axis force sensor of related art is the xe2x80x98Contact Force Sensorxe2x80x99 disclosed in JP-B-H.7-93445. In this contact force sensor also, piezoelectric sensors made by forming resistance devices on one side of an annular structural body made of a semiconductor are used, and semiconductor manufacturing process technology is utilized.
Of the multi-axis force sensors of these first through third examples of related art, whereas in the first multi-axis force sensor strain resistance devices (strain gauges) are affixed as external elements, in the second and third multi-axis force sensors, strain resistance devices are formed integrally on a semiconductor substrate by utilizing semiconductor device manufacturing process technology. The second and third multi-axis force sensors have the merit that they make it possible to resolve the problems associated with the first multi-axis force sensor.
However, related art multi-axis force sensors fabricated using semiconductor device manufacturing process technology have had the characteristic structurally that, when an attempt is made to detect a force or moment on each of three orthogonal axes, the whole substrate distorts isotropically in correspondence with the applied force or moment, and have also had the problem that the disposition of the multiple strain resistance devices on the substrate is not optimal and an external force acting on the distorting body part cannot be separated into components with good precision.
That is, in six-axis force sensors, there has been the problem that for example when an external force is applied so that only an axial stress component Fx arises, stresses arise and outputs are produced in connection with components other than Fx, which should properly be 0. There has been the problem that it is difficult to separate an external force applied from an unknown direction into individual components with good precision. The electrical signal components outputted from the resistance devices corresponding to the respective axis components superpose onto the other axes, and the measuring sensitivity of the axis components of force or moment decreases.
The problem of not being able to separate the axis components (forces and moments) of an external force acting on the distorting body part in a six-axis force sensor is known as the problem of xe2x80x98other axis interferencexe2x80x99. This problem of other axis interference is one which cannot be ignored from the point of view of realizing a practical six-axis force sensor.
The problem of other axis interference in a six-axis force sensor will now be explained more specifically, from a mathematical point of view, using equations.
In a six-axis force sensor, as mentioned above, as six axis components pertaining respectively to an X-axis, a Y-axis and a Z-axis, forces Fx, Fy and Fz and moments Mx, My and Mz are detected. The six-axis force sensor outputs six signals Sig1 to Sig6 (xe2x80x98computed resistance change proportionsxe2x80x99) using resistance changes of strain resistance devices provided on a distorting body part and on the basis of an external computing part. These six output signals Sig1 to Sig6 are associated with the six axis components Fx, Fy, Fz, Mx, My, Mz using 6xc3x976 matrix elements obtained by finding in advance the size (electrical change proportion) of each signal with respect to an input made by applying as an external force a single axis component only.
For a six-axis force sensor, the six axis components Fx, Fy, Fz, Mx, My, Mz will be written respectively as F1, F2, F3, F4, F5, F6 (generally, xe2x80x98Fi:i=1-6xe2x80x99). The above-mentioned six output signals Sig1 to Sig6 will be written S1, S2, S3, S4, S5, S6 (generally, xe2x80x98Si:i=1-6xe2x80x99).
Between Si and Fi above, expressed with a matrix equation (the symbols xe2x80x98( )xe2x80x99 in the equation indicating matrices), the following relationship holds.
(Si)=(mij)xc3x97(Fj) (j=1-6)xe2x80x83xe2x80x83(101)
That is, the equation (101) has the following meaning:
S1=m11.F1+m12.F2+m13.F3+m14.F4+m15.F5+m16.F6
S2=m21.F1+m22.F2+m23.F3+m24.F4+m25.F5+m26.F6 . . . (abbreviated)
S6=m61.F1+m62.F2+m63.F3+m64.F4+m65.F5+m66.F6
In equation (101), by finding in advance the computed respective resistance change proportions S1-S6 corresponding to the input of a single component only, it is possible to obtain the matrix elements mij of the matrix (mij). By calculating the inverse matrix (mij)xe2x88x921 of the obtained matrix (mij), the following equation is obtained:
(Fi)=(mij)xe2x88x921xc3x97(Sj)=(mxe2x80x2ij)xc3x97(Sj)xe2x80x83xe2x80x83(102)
The equation (102) has the following meaning:
F1=mxe2x80x211.S1+mxe2x80x212.S2+mxe2x80x213.S3+mxe2x80x214.S4+mxe2x80x215.S5+mxe2x80x216.S6
F2=mxe2x80x221.S1+mxe2x80x222.S2+mxe2x80x223.S3+mxe2x80x224.S4+mxe2x80x225.S5+mxe2x80x226.S6 . . . (abbreviated)
F6=mxe2x80x261.S1+mxe2x80x262.S2+mxe2x80x263.S3+mxe2x80x264.S4+mxe2x80x265.S5+mxe2x80x266.S6
In the above equation (102), xe2x80x9cmxe2x80x2ijxe2x80x9d is a matrix element of the 6xc3x976 inverse matrix (mij)xe2x88x921.
From the above equation (102), on the basis of the computed resistance change proportions (S1-S6) obtained from the resistance change proportions of the semiconductor strain resistance devices, it is possible to calculate the six axis components F1-F6 (the forces and moments of each axis direction).
In equations (101) and (102) above, if the values of the matrix elements mij, mxe2x80x2ij are all large, then for example when the computed resistance change proportions Si fluctuate due to the superposition of noise, that influence appears in the measurement values of F1-F6. That is, when there is an input consisting of a single component only as an external force, although the inputs of the other components are xe2x80x9c0xe2x80x9d, there is a high probability of the phenomenon arising of the measurement results not being xe2x80x9c0xe2x80x9d due to disturbances such as noise.
AS mentioned above, the obtained measured value of one of the six axis components, that is, forces or moments, fluctuating as a result of a force or moment of another axis is defined as xe2x80x98other axis interference occurringxe2x80x99.
Ideally, in the matrix (mxe2x80x2ij), the non-diagonal elements i.e. the elements other than the diagonal elements mxe2x80x211, mxe2x80x222, mxe2x80x233, mxe2x80x244, mxe2x80x255 and mxe2x80x266, should be xe2x80x9c0xe2x80x9d. In this case, the above-mentioned equation (102) becomes as follows:
F1=mxe2x80x211.S1
F2=mxe2x80x222.S2 . . . (abbreviated)
F6=mxe2x80x266.S6
It this relationship holds, the calculation becomes extremely simple, and other axis interference can be prevented.
In practice, even if the non-diagonal elements cannot be made xe2x80x9c0xe2x80x9d, if the values of the non-diagonal elements can be made extremely small compared to the diagonal elements, the problem of other axis interference can be reduced.
However, with the second and third multi-axis force sensors of related art mentioned above, because the whole substrate distorts and insufficient consideration has been given to the suitability of the disposition pattern of the semiconductor strain resistance devices, the non-diagonal elements of the matrix (mxe2x80x2ij) cannot be made xe2x80x9c0xe2x80x9d or made sufficiently small compared with the diagonal elements, and the probability of other axis interference occurring is high. Also, with the multi-axis force sensors mentioned above, due to other axis interference readily occurring, noise caused by unexpected disturbances and the like superposes on the electrical signals from the strain resistance devices, and consequently there is a high risk of measurement results fluctuating greatly with other axis interference as the cause. Consequently, when the second and third multi-axis force sensors mentioned above are used on a robot or the like, depending on their installation conditions, other axis interference causes their measured values to fluctuate, and when they are made general-purpose parts there are problems with reproducibility and robustness.
A fourth example of a multi-axis force sensor of related art is the xe2x80x98Micro-Manipulator Having Force Sensorxe2x80x99 disclosed in Japanese Unexamined Patent Publication No. H.11-333765. The force sensor disclosed in this document is fabricated using semiconductor manufacturing process technology, like the second and third related art examples mentioned above, and a three-component force sensor made up of a base and a central thick part and thin supporting parts connecting thereto and having strain sensors provided on the supporting part is shown.
To resolve the above-mentioned shortcomings of the multi-axis force sensors of the aforementioned second and third related art examples using semiconductor manufacturing process technology, in this fourth related art example, a structure is proposed wherein components of strain are separated axis by axis. However, although this structure achieves a slight improvement compared to other related art, it is a construction for performing detection of forces in the directions of three axes (X-axis, Y-axis, Z-axis), and when it is used as a six-axis force sensor its component-separating capability is inadequate and it cannot resolve the problem of other axis interference.
A further, fifth example of a multi-axis force sensor of related art is the xe2x80x98Sensorxe2x80x99 disclosed in JP-B-H.5-75055. This sensor is formed using a semiconductor substrate and has a central supporting body, a peripheral supporting body, and a plurality of connecting parts (beams) connecting these. According to FIG. 1 of this document, a resistance film pattern made up of multiple resistance devices is formed integrally with the surfaces of two predetermined connecting parts (beams) by film-forming technology. Because the connecting parts are parts of the semiconductor substrate, they have a thin plate shape. When the sensor receives a force on the central supporting body, the semiconductor substrate itself bends as a whole, and six force components are taken out by the multiple resistance devices provided on the connecting parts. In the sensor of this related art example also, there is a possibility of mutual interference among the detected six force components.
Another issue addressed by the invention will now be discussed. A six-axis force sensor fabricated using semiconductor manufacturing process technology contributes to sensor device size reduction. To make a six-axis force sensor small, the semiconductor substrate becomes small and becomes thin. In a six-axis force sensor formed using a semiconductor substrate, the semiconductor substrate itself functions as a distorting body. Consequently, there is a limit on the range of forces which can be measured, which depends on the basic strength of the semiconductor substrate. From the point of view of practical application, there is a need for a sensor to be designed so that this limit is not problematic, and for the measurement range to be raised to widen the range of applications.
It is therefore an object of the invention to solve the problems mentioned above and provide a six-axis force sensor with which in detecting an applied external force it is possible to suppress other axis interference, detect axis components of force and moment with high precision, and increase robustness and reproducibility.
It is another object of the invention to provide a six-axis force sensor which while suppressing other axis interference dramatically raises the detectable level of force, has a widened range of applications, and contributes to practical usability of six-axis force sensors.
To achieve these objects, a six-axis force sensor according to the invention has the following construction.
The six-axis force sensor according to the invention consists of a thin plate-shaped sensor chip formed using a substrate by semiconductor film-forming processes and having a six-axis force sensor function. The sensor chip has an action part for an external force to be applied to, a support part to be fixed to an external structure, and a number of connecting parts each having a high-rigidity bridge part joined to the action part and a low-rigidity elastic part joined to the support part and connecting together the action part and the support part. A number of strain resistance devices consisting of active layers are formed on at least one of the front and rear faces of each of the connecting parts in an area where deformation strain effectively occurs, and these strain resistance devices are electrically connected to corresponding electrodes provided on the support part.
The six-axis force sensor is a sensor chip having a six-axis force sensor function formed using a semiconductor substrate and film-forming technology. The connecting parts are each made up of a high-rigidity bridge part and a low-rigidity elastic part. Semiconductor strain resistance devices (Sya1-Sya3, Syb1-Syb3, Sxa1-Sxa3, Sxb1-Sxb3) consisting of active layers are disposed on the front sides of the connecting parts. The elastic i.e. low-rigidity parts of the connecting parts have the function of absorbing excess strains acting on the connecting parts and suppressing the occurrence of strains extending over the semiconductor substrate as a whole. Because forces and moments of specified axis components cause strains to occur selectively in corresponding strain resistance devices, by suitably combining measured results from selected resistance devices it is possible to effectively separate an applied external force into six components of force and moment and greatly suppress other axis interference in the measured results. As a result, a six-axis force sensor according to the invention can measure six components of force and moment of an external force applied to the action part with good reproducibility on the basis of the strains arising in the resistance devices disposed on the connecting parts
In the six-axis force sensor, the connecting parts are disposed with uniform spacing around the action part. The low-rigidity parts are elastic parts each connected to the support part at two locations. The high-rigidity parts are bridge parts connected to the action part. Accordingly, the elastic parts have the function of absorbing excess strains acting on the bridge parts and suppressing the occurrence of strains extending over the semiconductor substrate as a whole caused by the application of a force or moment in one direction, and because a force or moment in a given direction causes a strain selectively in a corresponding resistance device, other axis interference is greatly suppressed.
Preferably, the connecting parts are disposed with uniform spacing around the action part and so that adjacent connecting parts are mutually perpendicular.
Preferably, the action part is square and each of the connecting parts is formed in a T-shape made by its elastic part and its bridge part and is disposed at a respective one of the four sides of the action part. The strain resistance devices are disposed on an area of the surface of the bridge part near the boundary between the bridge part and the action part.
Preferably, the strain resistance devices are disposed on a narrowed portion formed in the bridge part and, more preferably, are disposed on the bridge part of the respective connecting part in parallel with the length direction of the bridge part and arranged side by side in a line in a direction perpendicular to the length direction of the bridge part. Resistance devices for temperature compensation can also be disposed on the support part.
In a six-axis force sensor according to the invention as described above, the strain resistance devices are disposed on the surf ace of the bridge part in an area where, when an external force is applied to the action part, greater strains arise than in the action part and other areas of the connecting part. As a result, the resistance devices can detect required components of force and moment selectively, the resistance changes arising in the resistance devices with respect to the external force can be made large, the precision with which the forces and moments of an external force applied to the action part are measured increases, and robustness and stability can be obtained in the measurement results.
Because the strain resistance devices are disposed on the surface of each of the connecting parts near the joining portion between its bridge part and the action part, they are disposed in the area where strains caused by the external force most concentrate. As a result, the changes in the resistance values of the semiconductor resistance devices arising from an external force can be made large.
If the resistance devices are disposed on a narrowed portion formed in the bridge part, because in the connecting parts corresponding to components of force and moment of an external force applied to the action part the greatest stresses arise in the narrowed part, strains concentrate most in this narrowed part, and the resistance changes of the resistance devices caused by the external force can be made large.
Disposing multiple resistance devices on each bridge part in parallel with the long axis direction of the bridge part makes it possible to combine resistance change proportions obtained from multiple resistance devices as electrical signals are taken out. This makes it possible to select resistance devices so that other axis interference is prevented, i.e. so that resistance change proportions pertaining to forces and moments in other than a certain direction cancel each other out, and makes it possible to compose optimal formulas for obtaining six components of force and moment from resistance change proportions of these resistance devices. On the basis of these formulas, the non-diagonal elements in a matrix expressing the relationship between forces and moments and resistance change proportions can be made xe2x80x9c0xe2x80x9d or amply small.
By disposing semiconductor resistance devices for temperature compensation on the support part and interconnecting them to corresponding electrodes it is possible to detect resistance changes due to the temperature of the surroundings. By correcting measured results of resistance change in correspondence with the temperature characteristics of the semiconductor resistance devices on this basis it is possible to obtain force and moment measurements unaffected by the ambient temperature.
In the six-axis force sensor as arranged above, preferably, a guard interconnection at a ground potential is provided so as to surround the non-ground interconnections for the strain resistance devices. In this case, in the detection of currents from the resistance devices, disturbances caused by high-frequency noise are suppressed, and because the guard interconnection shields against cross-talk from the interconnections of other resistance devices, the S/N-ratio of current measurement of resistance changes of the resistance devices can be increased, the precision with which component forces and moments of an applied external forces are measured can be increased, and robustness and stability are obtained in the measurement results.
In the six-axis force sensor as arranged above, preferably, a bias electrode for impressing a bias voltage on the substrate is provided. With this construction, by means of an impressed bias, it is possible to grow a depletion layer at the active layer interfaces and thereby to effect insulation between the active layers and the semiconductor substrate and between adjacent active layers, and consequently leak currents decrease and current noise influences can be reduced. Also, by electrically fixing the substrate at a fixed potential, variation of potential can be prevented and resistance to noise can be increased, and resistance changes in the semiconductor resistance devices can be measured with high precision; consequently, the measurement precision of component forces and moments of an applied external force can be increased, and robustness and stability can be obtained in the measurement results.
Preferably, at least one of the strain resistance devices on each of the connecting parts is disposed on the rear side of the substrate in the area corresponding to the area on the front side of the substrate where the strain resistance devices are disposed on the bridge part. By means of this construction, on the basis of resistance changes of resistance devices formed on both the front side and the rear side of the substrate, an external force applied to the action part can be better resolved into specific force and moment components compared to when resistance devices are provided only on the front side of the substrate, and more robustness and stability can be obtained in the measurement results.
In this six-axis force sensor, the internal corners at the joining portions of the connecting parts and the action part, the joining portions of the connecting parts and the support part, and the joining portions of the elastic parts and the bridge parts, are all worked to a circular arc shape. With this construction, stresses at the joining portions are dispersed, concentration of stress at the internal corners arising from the external force applied to the action part is suppressed, structural strength is increased, and the measurable range of applied external forces can be widened.
Another six-axis force sensor provided by the invention has a thin plate-shaped sensor chip formed using a substrate by semiconductor film-forming processes and having a six-axis force sensor function and including at least an action part for receiving an external force and a support part supporting the action part, and a structural body provided around the sensor chip and including an external force application part to which an external force is applied, a plinth part for supporting the sensor chip, an external force buffering mechanism fixing the external force application part to the plinth part, and an external force transmitting part, and the external force application part and the action part are linked by the external force transmitting mechanism.
In this six-axis force sensor, a sensor chip made using a semiconductor substrate is provided with a structural body made up of an external force application part, a plinth part, an external force buffering mechanism and an external force transmitting mechanism, so that the external force is attenuated as it acts on the sensor chip. By this means it is possible, while suppressing the problem of other axis interference in a six-axis force sensor, to raise the load-withstanding capability of the sensor, raise its dynamic range, enlarge its measurable range of external forces, and raise its practical usability. Also, a buffering mechanism for the six-axis force sensor is realized with a simple structure, and by optimizing this structure it is possible to adjust the load-withstanding characteristics of the sensor. It is possible to manufacture a six-axis force sensor which is resistant to external noise, has high reproducibility of detection performance, can be fabricated without dispersion arising in its performance, and is optimized with respect to one of various force ranges.
In the construction described above, preferably, the sensor chip has an action part, a support part, four connecting parts, and multiple strain resistance devices fabricated by semiconductor film-forming processes on parts where deformation will occur. An insulating member can be provided between the sensor chip and the plinth part.
Because the sensor chip has four connecting parts and multiple strain resistance devices in a predetermined disposition pattern, this six-axis force sensor has a detection performance such that other axis interference is suppressed as far as possible. Because an insulating member is interposed between the sensor chip and the plinth part, electrical noise can be prevented from passing from the structural body having the buffering mechanism to the sensor chip.